Rock Mechanics - Lec 10

Rock Mechanics

• “The theoretical and applied science of the mechanical behaviour of rock and rock masses in response to force fields of their physical environment.”
• Engineering Rock Mechanics
• Geological Rock Mechanics

Rock Features

• Strength of the rock is dependent on its physico-chemical composition.
• Physico-mechanical characteristics of same rock type may vary
• Rocks are strong in taking compressive load but relatively weak in taking tensile load
• Rocks behave mostly as brittle material but may behave as ductile/plastic

Porosity

• Intrinsic property
• The storage capacity of the geologic material
• igneous or metamorphic rocks (0-2%)
• sedimentary rocks (up to 40%)

Primary Porosity

Ranges from < 1% in crystalline rocks like granite to over 55% in some soils.

Secondary Porosity

Porosity of some rock is increased through fractures or solution of the material itself

Void Index

• Mass of water contained in a rock sample after one hour period of immersion
• Ratio of difference between the saturated and dry weight of rock to dry weight of rock

Permeability

• Relative interconnection of pores
• “If the material has high permeability than pore spaces are connected to one another allowing water to flow from one to another, however, if there is low permeability then the pore spaces are isolated and water is trapped within them.”

Sonic Velocity

• Evaluates the degree of fissuring
• Uniaxial Compressive Strength (UCS) testing estimating in situ rock strength in coal mines

Durability

• Inverse of the alterability of rocks
• Controlling not only the stability of slopes, surface and underground excavations but also that of a great number of stone structures, buildings and monuments.

Elastic

• Change is reversible
• Elasticity
• Shale, milestone

Ductile

• Change is not reversible
• Ductility
• Clay, Mica

Fracture

• Change is irreversible and the rock breaks
• Brittle
• Quartz, Feldspar

Deformation

• Elastic
• Ductile
• Fracture
Other factors:
• Rate of Stress
• Temperature
• Confining Pressure

Stress

• Compression
• Tension
• Shear
• _____ causes Strain

Compression

Pressed together

Tension

Stretched apart

Shear

Slips horizontally

Strain

• Amount of deformation
• _____ results in Structure

Strength

• Existing competency of the rock fabric binding components.

Laboratory Tests

1) Unconfined Compression Tests
2) Triaxial Compression Test
3) Splitting Tension Test
4) Beam Bending Test
5) Ring Shear test

Properties of Rocks for Engineering Purposes

1. Strength of rock

2. Drill core quality

3. Joint spacing

4. Joint characteristics

5. Ground water conditions

6. Orientation of joints

Importance of opportunities

Minimum risks during construction
Speed of Construction
Long term quality of infrastructure

Physical Properties

Mineralogical structure, texture.
composition
mineral
Specific gravity, density, unit weight
Porosity, void ratio
Moisture content, degree of saturation
Permeability
Swelling properties
Anisotropy
Electrical properties
Thermal properties
Velocity of Elastic waves
Durability

Mechanical Properties

Elastic
Modulus/Deformation
modulus and Poisson's ratio
Uniaxial compressive strength
Tensile strength
Shear strength Properties
Point load strength
Rock hardness

High porosity

Highly porous marine sediments
Unconsolidated sediments
Sandstones
Carbonate rocks
Low porosity
Fractured igneous rocks, other dense rock types

Compressive Strength of Rock

• Unconfined compressive strength
qu = 23 Is
I s = point load strength